Site does not run when Refreshing personalization

Hi,

I am currently follwing post: https://world.episerver.com/documentation/developer-guides/find/personalized-find/

I have added in the appropriate app settings for episerver:personalization.Site and episerver:personalization.ClientToken and have added

UsingPersonalization() extension method to the search query. Up this point the site runs fine.

However when I add the Controller Attribute:

[EPiServer.Find.Personalization.RefreshPreferences]

or use the below code to refresh the personalization:

System.Threading.Tasks.Task.Factory.StartNew(currentContext => { 
System.Web.HttpContext.Current = (HttpContext)currentContext; 
_client.Personalization().Refresh();
 }, System.Web.HttpContext.Current.ApplicationInstance.Context);

my site won't wont run at all and gives the following object reference error:

[NullReferenceException: Object reference not set to an instance of an object.]
   EPiServer.Find.Personalization.ClientExtensions.Personalization(IClient client) +22
   EPiServer.Find.Personalization.RefreshPreferencesAttribute..ctor() +43
   System.RuntimeTypeHandle.CreateCaInstance(RuntimeType type, IRuntimeMethodInfo ctor) +0
   System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeModule decoratedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFilterType, Boolean mustBeInheritable, IList derivedAttributes, Boolean isDecoratedTargetSecurityTransparent) +1476
   System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeType type, RuntimeType caType, Boolean inherit) +160

Any help as to why this would be greatly appreciated!

Regards,

Martin